Field of the Invention
The present invention relates generally to an apparatus for refining operations, and more particularly, but not by way of limitation, to delayed coking operations utilizing a heater coil having straight tubes constructed of a first material and return bends constructed of a second material wherein the first material and the second material exhibit differing thermal properties, in particular, but not by way of limitation, design-maximum tube-metal temperatures.
History of the Related Art
Delayed coking refers to a refining process that includes heating a residual oil feed, made up of heavy, long-chain hydrocarbon molecules, to a cracking temperature in a furnace. Typically, furnaces used in the delayed coking process include a plurality of tubes arranged in a multiple-pass configuration. Heating of the residual oil feed cracks the heavy, long-chain hydrocarbon molecules producing gas, lightweight products, and solid coke. The gas and lightweight products are further refined into various liquid fuels and gas fuels. The solid coke is subsequently crushed and sold as a fuel source.
During the delayed coking process, solid coke forms on an inside surface of the plurality of tubes. This phenomenon is known as “fouling.” Solid coke is an insulator and causes a temperature of a material forming the plurality of tubes (referred to herein as a “tube-metal temperature”) to incrementally increase during operation. For example, a clean tube may require a tube-metal temperature of, for example, 945° F. to heat the residual oil feed to 900° F. In contrast, a fouled tube might require a substantially higher tube-metal temperature to heat the residual oil feed to 900° F. Over a period of use, the plurality of tubes eventually reach a design-maximum tube-metal temperature. As used herein, the term “design-maximum tube metal temperature” refers to a maximum safe operating temperature of the plurality of tubes. Above the design-maximum tube metal temperature, thermal stresses can contribute to wear and fatigue of the plurality of tubes thereby rendering the furnace unsafe for operation. Upon reaching the design-maximum tube-metal temperature, the plurality of tubes must be cleaned to remove the solid coke. Cleaning brings the plurality of tubes back to the tube-metal temperature conditions associated with a clean tube.
Cleaning the plurality of tubes typically involves at least one of mechanical cleaning, steam-air decoking, pigging, or online spalling. Online spalling involves removing a fouled pass including a plurality of tubes from service and thermally shocking the plurality of tubes. The plurality of tubes are rapidly heated (expanded) and cooled (contracted) over a set period of time. During cooling, the fouled tube contracts causing a portion of the solid coke accumulated therein to break free. The solid coke is flushed out of the fouled tube and processed in a coke drum. The advantage of online spalling is that only one pass is spalled at a time allowing remaining passes to operate normally. However, the efficacy of online spalling may decrease each time it is performed.
Pigging involves passing a foam or plastic “pig” having metal studs and grit through the tube. As the pig passes through the fouled tube, the pig rotates and scrapes the solid coke from an inside surface of the fouled tube. Steam-air decoking involves circulating a steam-air mixture through the plurality of tubes at elevated temperatures. Air from the steam-air mixture is used to burn the solid coke from the inside surface of the plurality of tubes while steam from the steam-air mixture ensures that the burning temperatures do not exceed the design-maximum tube-metal temperature.
In most cases, during cleaning, at least a pass of the plurality of tubes must be removed from the residual oil feed. In some cases, the entire furnace must be taken out of service. This results in a reduction of productivity and a loss of profits. Thus, it is of great importance to design the furnace to maximize a period of time between cleanings.
U.S. Pat. No. 7,670,462, assigned to Great Southern Independent L.L.C., relates to a system and method for on-line cleaning of black oil heater tubes and delayed coker heater tubes. A high-pressure water charge is injected through the heater tubes during normal process operations to prevent heater tube fouling and downtime. The water charge undergoes intense boiling and evaporation. The intense boiling induces a scrubbing action within the heater tubes. Furthermore, a shocking action is induced by expansion and contraction of the heater tubes resulting from the water charge flowing through the heater tubes followed by a hotter process fluid flowing through the heater tubes.
U.S. Patent Application Publication No. 2007/0158240, assigned to D-COK, LP relates to a system and method for on-line spalling of a coker. An off-line heater pipe is added to on-line coker heater pipes. When an on-line pipe is to be spalled, flow is diverted to the off-line pipe thus allowing for full operation of the coker heater.